This invention relates generally to a thermally activated low-resistance permanent battery cell bypass around a failing or open cell. The technology is designed for use with NiH2 battery cells commonly used on spacecraft. The invention may be used in other applications as well. The bypass for high capacity (&gt;100 Ah) NiH.sub.2 cells should be a low dissipation switch. There are several existing patents for this type of device. All of the devices operate from the current supplied by the battery, shunting through the device, which activates the switch to close the circuit and bypass the cell. The designs typically fall into one of the three following categories:
1. A semiconductor (purely electronic) switch. PA1 2. A mechanical (fuse or relay) switch. PA1 3. A thermally activated semiconductor (thermal, shape memory metal, or low melting point metal alloy) switch.
The thermally activated switch utilizes a semiconductor device during normal operation, so that it never experiences an open circuit condition. An early example of this technology uses a diode in parallel with the cell and a thermal switch connected to a relay (Hall, Thomas, Hughes, 1993, Reversible Automatic Cell Bypass Circuit, U.S. Pat. No. 5,258,244). When the cell fails, the current heats the diode, activating the thermal switch, which activates a relay. The concerns with this design are high cost and complexity, and long-term mechanical integrity of the contacts over time and thermal cycling. Another early example a "thermally-activated, shorting diode switch", which operates based on the cell current heating the diode, then a solder preform melting across the diode to create a short-circuit (Herrin, Jerry, Hughes, 1988, Thermally-activated, Shorting Diode Switch having Non-operationally-alterable Junction Path, U.S. Pat. No. 4,774,558). This device apparently was never used in an application, and other than being somewhat complicated, it is the most similar design to the invention.
Martin Marietta (now Lockheed Martin) patented a novel cell bypass circuit using diodes to heat a shaped memory metal (SMM) rod which expands to close the contact (Kawam, Elias, Martin, 1996, Electro-thermally Actuated Switch, U.S. Pat. No. 5,510,598). The problem with this device is complexity and relatively high cost due to special plating requirements for the gold contact. ESTEC, AEA, and Matra Marconi Space have designed a thermally activated switch using diodes to heat a low melting point metal to create a short-circuit connection with a low-resistance contact (Dudley, G. J. et. al., ESA-ESTEC, 1998, Development and Pre-qualification Testing of a Low-dissipation Battery Cell Open Circuit Protection Device, French Patent?). The advantages of this design include a strong immunity to false triggering and a "self-healing" characteristic.